The stopping of drives for elevators is technically relevant from a safety point of view. When considering the functional course of elevators, the stopping after activation of a safety device and the unintentional starting during loading or unloading, respectively, are particularly essential.
In order to take these demands into account, the current supply to the motor is realised by means of two monitored relays or one monitored relay and one monitored control device, which interrupts the power supply by means of static components. This ensures that in the operation states mentioned above the motor can create no torque and the brake is engaged.
To this, DIN EN 81-1, point 12.7, states as follows:
The stopping of the elevator on activation of an electrical safety device must take place as follows:
In motors, which are supplied directly by the AC or DC mains, the power supply must be interrupted by two mutually independent relays, whose switching elements are connected in series in the motor current circuit.
If the main switching element of one of the relays has not opened when the elevator has stopped, a renewed starting must be prevented before the next direction change.
With a drive according to the Ward-Leonard system and generating the activation by classical means, two mutually independent relays must interrupt either:                a. the rotor circuit        b. the energizing circuit of the generator        c. one relay interrupts the rotor circuit and the other interrupts the energizing circuit of the generator.        
When the main armature of one of the two relays does not open when the elevator stops, a renewed starting must be prevented before the next direction change.
With a supply and control of AC or DC motors with static means, the power supply to the motor must be interrupted by two mutually independent relays. When the main armature of one of the two relays does not open when the elevator stands still, a renewed starting must be prevented before the next direction change.
Alternatively, a circuit comprising:                1. a relay, which interrupts the power supply on all poles. The coil of the relay must be turned off at least before each change of operation direction. When the relay does not open, a renewed starting of the elevator must be prevented        2. a control device that interrupts the power supply in the static elements        3. a monitoring device that tests if the power supply is interrupted at each stop of the elevatormust be provided.        
During the trade fair SPS//PC/DRIVES 2002 a new system from the company Control Techniques, the Unidrive SP, was presented, which is intended to be an automation platform provide a number of new, innovative solutions for the elevator business. A related article on the subject in the magazine LIFT-REPORT, 29th volume (2003), No. 4, page 80, ends with the statement: “A TÜV approval according to EN 81-1 is in progress. This will permit saving one motor relay.”
This outlined state of the art makes it clear that experts consider the motor protection principle as indispensable. This is in spite of the fact that state of the art involves substantial disadvantages.
Particularly with elevators without machine room, the space requirements and the noise generation of the relays to be used are disturbing. The high switching cycle prevents the use of a switching relay at the input of the frequency converter. Thus, it is difficult to locate the frequency converter directly at the motor. The costs of the relays, their mounting and wiring increase the manufacturing costs.
From an EMV point of view, the switching of the frequency converter outlet and thus the interruption of the screening is bad. It is also known that switching off the converter outlet at low motor frequencies generates higher contact erosion, which again causes a shorter life of the relays.
It is the task of the invention to eliminate these disadvantages and completely abandon the principle of using motor relays.